1.Field of the Invention
The present invention relates to a display, and more particularly, to an organic electroluminescent (EL) device.
2. Background of the Related Art
Organic EL devices, also called organic light emitting diodes (LEDs), are becoming very popular because of their possible application to flat panel displays (FPDs). Organic EL devices are extremely thin, matrix-addressable and operable at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional features suitable for next generation FPDs such as, among other things, little dependence on viewing angle and good device-formability on flexible substrates. Organic LEDs differ fundamentally from conventional inorganic LEDs. For Example, the charge transfer in inorganic LEDs is band-like in nature and the electron-hole recombination results in the interband emission of light; while organic films are generally characterized by low-mobility activated hopping transport, and the emission is excitonic. Furthermore, organic EL devices are substantially different from conventional inorganic EL devices in other respects, notably in that in that organic EL devices are operable at low DC voltages.
Referring to FIG. 1, a typical organic EL device is shown with a first electrode 2 formed on a transparent substrate 1, a hole injecting layer (HIL) 3 and a hole transporting layer (HTL) 4 formed on the first electrode 2, a luminescent layer 5 formed on the HTL 4, an electron transporting layer (ETL) 6 and an electron injecting layer (EIL) 7 formed on the luminescent layer 5, and a second electrode 8 formed on the EIL 7. Any one or more of HIL 3, HTL 4, ETL 6 and EIL 7 may be omitted, depending on the particular device structure adopted.
Electrons and holes injected into the luminescent layer through the second electrode 8 and the first electrode 2, respectively, recombine to decay radiatively. For most organic EL devices, the charge injection barrier is higher for electrons than for holes. It is well known that the electron injection barrier may be lowered by employing a low work function material for the second electrode 8. However, low work function materials are chemically reactive, which makes it difficult to use such materials for electrodes. Accordingly, such materials are often used as a second electrode after being alloyed with one of more stable materials, as seen in the examples of Mg:Ag and Al:Li. However, such alloyed second electrodes are still less stable, more costly to form, and more difficult to deposit in a uniform film as compared to aluminum.
An even more serious problem often encountered with an alloyed second electrode of Mg:Ag or Al:Li is the frequent occurrence of cross talk or current leakage between pixels, which may be attributed to the diffusion of Mg or Li ions across organic layers of the device. This problem can be greatly alleviated if one selects aluminum as a second electrode material. However, in the case of aluminum there is a need to improve its poor electron injecting capability. The electron injecting capability of a high work function second electrode, such as aluminum, can be significantly enhanced by inserting a very thin layer (typically 0.3 nm xcx9c1.0 nm) of an electrically insulating material such as LiF, MgF2 or Li2O, inserted either between an aluminum electrode and the luminescent layer, or between the aluminum electrode and the ETL(see, for example, IEEE Transactions on Electronic Devices, Vol. 44, No. 8, p 1245-1248(1997), the contents of which are incorporated herein in their entirety).
Li2O is a particularly interesting material, in this regard, in that it is an electrically insulating material with a very low work function. The work function of alkali metals themselves is very low, and it becomes even lower when oxidized: for example, work function decreases from 2.1 eV for Cs to about 1 eV for Cs2O. Various alkali metal compounds have reportedly been used to form an insulating buffer layer for the purpose of lowering the electron injecting barrier: e.g., Li2O, LiBO2, NaCl, KCl, K2SiO3, RbCl, and Cs2O to name a few. Despite this improvement, the introduction of the insulating buffer layer poses a challenging new problem, namely, deterioration of adhesion between an EL multilayer and aluminum, with consequent reduction of life time of the device. Experimental results reveal evidence of poor adhesion either at the buffer layer/aluminum interface or at the EL multilayer/buffer layer interface. This situation is not unexpected, given the different characteristics of materials involved. In summary, organic EL devices of the related art have at least two basic drawbacks, namely, poor adhesion and short life time.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
Accordingly, the present invention is directed to an organic electroluminescent device that substantially obviates one or more of the problems, limitations and/or disadvantages of the related art.
An object of the present invention is to provide an organic EL device with a long life time and have high efficiency.
To achieve these and other advantages, and in accordance with the present invention as embodied and broadly described herein, the organic electroluminescent device comprises at least one organic EL multilayer disposed between a first electrode and a second electrode, and a layer I, disposed between the organic EL multilayer and the second electrode, including at least one first material from the group consisting of compounds of chemical formula I: 
where:
each of R1 to R4 is independently hydrogen, an alkyl or alkoxy group having from 1 to 5 carbon atoms, aryl, aryloxy or a halogen, or at least one among pairs of adjacent substituents of R1 through R4 may form an five or six- numbered conjugated cyclic ring which may includes carbon, nitrogen, or sulfur; and,
A each independently denotes hydrogen, an alkyl group having from 1 to 5 carbon atoms, or aryl.
Herein, the term the organic EL multilayer may encompass a plurality of layers comprising a luminescent layer and typically one or more of the HIL, HTL, ETL, and EIL.
In a preferred form of the invention, said each of R1 to R4 is independently hydrogen, an alkyl and alkoxy group having from 1 to 5 carbon atoms, phenoxy, phenyl, naphthyl, fluorine, chlorine, or bromine, and said A each independently denotes methyl, ethyl, phenyl, or hydrogen.
In a highly preferred form of the invention, said R1 is an alkyl and alkoxy group having from 1 to 5 carbon atoms, phenoxy, or phenyl, and said each of R2 to R4 and A is independently hydrogen.
In addition, the above-mentioned organic electroluminescent device may further comprise a layer II including at least one second material selected from the group consisting of an alkali metal, an alkaline earth metal, and a compound thereof, which may be disposed between the layer I and the second electrode. Preferably, said second electrode may comprise aluminum. Also, said layer II may comprise Li2O. Said layer I may have a thickness of from about 0.5 nm to about 50 nm, and said layer II may have a thickness of from about 0.2 nm to about 3 nm.
According to another aspect of this invention, there is provided a means to improve the life time of an organic EL device, as well as the electron injecting capability, by inserting, instead of a dual layer of the layer I and the layer II, a mixed layer comprising a mixture of the components of layer I and layer II, wherein the mixed layer is inserted between the organic EL multilayer and the second electrode. The mixed layer is formed by the co-deposition of (1) at least one first material selected from the group consisting of compounds of chemical formula I and (2) at least one second material selected from the group consisting of an alkali metal, an alkaline earth metal, and a compound thereof. The ratio between the first and second materials in the mixed layer can be either fixed or varied as a function of position, i.e., by forming a concentration gradient of the first and second material within the mixed layer. Preferably, said mixed layer may have a thickness of from about 0.5 nm to about 10 nm. Also, said second electrode comprises aluminum. Said second material may comprise Li2O.
The present invention is also directed to an organic electroluminescent (EL) device comprising: at least one organic EL multilayer between a first electrode and a second electrode, and a layer I, disposed between the organic EL multilayer and the second electrode, including at least one first material selected from porphyrinic compounds. Herein, the term the organic EL multilayer may encompass a plurality of layers comprising a luminescent layer and typically one or more of the HIL, HTL, ETL, and EIL. The layer I includes at least one porphyrinic compound and serves principally to improve adhesion between the organic EL multilayer and the second electrode, while retaining good electron transporting capability.
Preferred porphyrinic compound according to the invention are those of the structural formula II as shown below. 
where:
A each independently denotes xe2x80x94Nxe2x95x90 or xe2x80x94C(R)xe2x95x90, and R is hydrogen, alkyl, alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;
M comprises an element selected from groups IA, IIA, IIIA and IVA, and the third, fourth, fifth and sixth periods of the periodic table;
Y is alkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group, an arylphosphinic group, alkylsulfur or arylsulfur, or an element selected from groups VIA and VIIA of the periodic table;
n is an integer of 0, 1, or 2; and,
B1 through B8 each independently represents hydrogen, alkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino, arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group, or at least one among pairs of adjacent substituents of B1 through B8 may form an unsaturated or saturated five, six, or seven-numbered ring which may include substituents such as alkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino, arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group. Preferred five, six or seven-numbered rings are those which include carbon, sulfur, oxygen or nitrogen ring atoms.
More highly preferred examples of useful porphyrinic compounds are phthalocyanines. Exemplary preferred materials are those of structural formulas III and IV as shown below: 
where:
A each independently denotes xe2x80x94Nxe2x95x90 or xe2x80x94C(R)xe2x95x90; and R is hydrogen, alkyl, alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;
M comprises an element selected from groups IA, IIA, IIIA and IVA, and the third, fourth, fifth and sixth periods of the periodic table;
Y is alkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group, an arylphosphinic group, alkylsulfur, or arylsulfur, or an element selected from groups VIA and VIIA of the periodic table;
n is an integer of 0, 1, or 2; and,
X1 through X8 each independently represent hydrogen, alkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino, arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group, or at least one among pairs of adjacent substituents of X1 through X8 may form an unsaturated or saturated five, six, or seven-numbered ring which may include substituents such as alkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, akylamino, arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group. Preferred five, six or seven-numbered rings are those which include carbon, sulfur, oxygen or nitrogen ring atoms.
The most preferred examples of phthalocyanines are those of structural formulas V and VI as shown below: 
where:
M is one of Co, AlCl, Cu, 2Li, Fe, Pb, Mg, SiCl2, 2Na, Sn, Zn, Ni, Mn, VO, 2Ag, MnCl, SnCl2, and TiO.
In addition, the above-mentioned organic electroluminescent device may further comprise a layer II including at least one second material selected from the group consisting of an alkali metal, an alkaline earth metal, and a compound thereof.
According to another aspect of this invention, there is provided a means to improve the life time of an organic EL device, as well as the electron injecting capability, by inserting, instead of a dual layer of the layer I and the layer II, a mixed layer comprising a mixture of the components of layer I and layer II, wherein the mixed layer is inserted between the organic EL multilayer and the second electrode. The mixed layer is formed by the co-deposition of (1) at least one first material selected from the group consisting of compounds of chemical formula II and (2) at least one second material selected from the group consisting of an alkali metal, an alkaline earth metal, and a compound thereof. The ratio of the first and second materials in the mixed layer can be either fixed or varied as a function of position, i.e., by forming a concentration gradient of the first and second materials within the mixed layer.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.